The present invention relates generally to gas generating compositions used for inflating occupant safety restraints in motor vehicles, and more particularly to nonazide gas generants that produce combustion products having acceptable toxicity levels in the event of exposure to vehicle occupants.
Inflatable occupant restraint devices for motor vehicles have been under development worldwide for many years, including the development of gas generating compositions for inflating such occupant restraints. Because the inflating gases produced by the gas generants must meet strict toxicity requirements, most, if not all, gas generants now in use are based on alkali or alkaline earth metal azides, particularly sodium azide. When reacted with an oxidizing agent, sodium azide forms a relatively nontoxic gas consisting primarily of nitrogen. Moreover, combustion of azide-based gas generants occurs at relatively low temperatures, which enables the production of nontoxic inflating gases without a need for additives to reduce the combustion temperature.
However, azide-based gas generants are inherently difficult to handle and entail relatively high risk in manufacture and disposal. Whereas the inflating gases produced by azide-based gas generants are relatively nontoxic, the metal azides themselves are conversely highly toxic, thereby resulting in extra expense and risk in gas generant manufacture, storage, and disposal. In addition to direct contamination of the environment, metal azides also readily react with acids and heavy metals to form extremely sensitive compounds that may spontaneously ignite or detonate.
Another problem inherent in azide-based gas generants is the production of very fine toxic powders upon combustion. Exemplary of these very fine toxic powders are ions and oxides of alkali or alkaline earth metals, such as sodium metal or sodium peroxide, depending on which metal azide is utilized in the gas generant. These very fine toxic residues have been heretofore removed from the inflating gases produced by the azide-based gas generant by incorporating a low-temperature softening glass into the azide-based gas generant, as described in U.S. Pat. No. 4,021,275. The glass acts as a secondary media filter in order to remove the very fine toxic powders. In a first phase, the glass melts and absorbs dispersed toxic powders. In a second phase, the molten glass adheres to a primary filter, such as a wire net or mesh, and facilitates accretion of the fine toxic powders onto the primary filter mesh or net.
In contradistinction, nonazide gas generants provide significant advantages over azide-based gas generants with respect to toxicity related hazards during manufacture and disposal. Moreover, most nonazide gas generant compositions typically supply a higher yield of gas (moles of gas per gram of gas generant) than conventional azide-based occupant restraint gas generants.
However, nonazide gas generants heretofore known and used produce unacceptably high levels of toxic substances upon combustion. The most difficult toxic gases to control are the various oxides of nitrogen (NO.sub.x) and carbon monoxide (CO).
Reduction of the level of toxic NO.sub.x and CO upon combustion of nonazide gas generants has proven to be a difficult problem. For instance, manipulation of the oxidizer/fuel ratio only reduces either the NO.sub.x or CO. More specifically, increasing the ratio of oxidizer to fuel minimizes the CO content upon combustion because the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NO.sub.x. Alternatively, if the oxidizer/fuel ratio is lowered to eliminate excess oxygen and reduce the amount of NO.sub.x produced, increased amounts of CO are produced.
The relatively high levels of NO.sub.x and CO produced upon combustion of nonazide gas generants, as opposed to azide-based gas generants, are due primarily to the relatively high combustion temperatures exhibited by nonazide gas generants. For example, the combustion temperature of a sodium azide/iron oxide gas generant is 969.degree. C. (1776.degree. F.), while the nonazide gas generants exhibit considerably higher combustion temperatures, such as 1818.degree. C. (3304.degree. F.). Utilizing lower energy fuels to reduce the combustion temperature is ineffective because the lower energy fuels do not provide a sufficiently high gas generant burn rate for use in vehicle occupant restraint systems. The burn rate of the gas generant is important to ensure that the inflator will operate readily and properly.
Another disadvantage created by the high combustion temperatures exhibited by nonazide gas generants is the difficulty presented in forming solid combustion particles that readily coalesce into a slag. Slag formation is desirable because the slag is easily filtered, resulting in relatively clean inflating gases. In azide-based gas generants, the lower combustion temperatures are conducive to solid formation. However, many common solid combustion products which might be expected from nonazide gas generants are liquids at the higher combustion temperatures displayed by nonazide gas generants, and are therefore difficult to filter out of the gas stream.
Therefore, a need exists for a nonazide gas generant that can produce inflating gases at a desired high burn rate but at a relatively low combustion temperature so that toxic gases, for example, NO.sub.x and CO are minimized.